Wet weather returns this week especially through Wednesday; no more summer heat, but still relatively humid with lack of cool nights and unusually little temperature variation; Hurricane Lee may impact eastern New Hampshire and Maine next Sunday, then possible warmup for the following week

Plain-language summary:

This week will mark a return to the wet pattern that has been familiar for most of the summer. A weak storm will lead to clouds and showers dominating in northern New England today and tomorrow, with drier conditions in Ontario and Quebec with increasing sunshine tomorrow. After a brief break Monday night into Tuesday, a stronger storm tracking farther northwest will bring clouds and rain first to areas northwest of the St. Lawrence River later Tuesday into Tuesday night, but then spreading to the rest of the region by Wednesday before slowing moving out and drying out by Thursday perhaps except in Maine, where it may take into Friday to dry out. After a dry Friday, Hurricane Lee could impact eastern New Hampshire and Maine next Sunday or the following Monday, but there is considerable uncertainty in if Lee goes that far west or hits Atlantic Canada instead, depending on the location and speed of the various weather features including Lee itself. With clouds and humidity through Wednesday, day-to-night temperature fluctuations will be smaller than usual, and even beyond that, day-to-day temperature fluctuations will be smaller than usual for the foreseeable future with a lack of particularly hot or cold air masses.

Meteorological discussion:

 

After a hot and dry spell earlier in the week that marked the longest stretch of dry weather since late May or early June in most locations, this week will be the opposite, more akin to earlier in the summer, with a chance of rain on most days, though somewhat depending on location. A weakening cold front stalled over southeastern New England yesterday, and a weak low pressure is now moving northeastward along it, bringing clouds and intermittent rain across most of northern New England today and tonight, though Ontario and Quebec remain mostly dry. The low pressure will move off to the east by later tomorrow, though it will still be showery and cloudy for most of northern New England, with increasing sunshine in Ontario and Quebec. Rainfall amounts will be 0.4-1.2” (10-30 mm) for most of northern New England.

 

Source: TropicalTidbits and PivotalWeather

 

After a dry period Monday night into Tuesday, a stronger low pressure approaches from the west, this time taking a track to the northwest over northeastern Ontario and northwestern Quebec, as its associated upper-level trough over the Great Lakes cuts off and moves very slowly. This is accompanied by the +PNA and ridge in western North America that will dominate this week, as well as the -NAO that will provide high-latitude blocking, a familiar combination this summer. This will provide deep southerly flow of rich Atlantic moisture into our area. Initially, most of the rain and clouds will be northwest of the St. Lawrence Valley, closer to the low pressure, but by Wednesday, scattered showers and maybe even thunderstorms will occur throughout the region with abundant moisture and a bit of instability. However, given the cloud cover and the time of year, severe thunderstorms are not expected, with heavy rainfall over already wet ground from recent rains being the main threat. The system will shift enough to the east for drier westerly flow to return by Thursday, possibly except for Maine, where it may not dry out until early Friday. The timing is still a bit uncertain though. A widespread 0.8-1.6” (20-40 mm) of rain is expected, with locally up to 75 mm in the upslope favored areas, especially the southern slopes of the White Mountains. There might be less rain in eastern Ontario and just northwest of the St. Lawrence River, which will be slightly removed from the best moisture.

 

Source: TropicalTidbits

 

Source: PivotalWeather

 

The timing of the cut-off low, the speed of Hurricane Lee, and the location of then likely Hurricane Margot will be critical in determining if Lee will affect the weather in our region. Most indications are that the cut-off low will move off to the east too early to capture Lee, which will still be southwest of Bermuda by then. This allows a ridge to build over Atlantic Canada northeast of Lee, preventing it from accelerating very quickly but instead steering it gradually to the north. Margot might also help build the Atlantic Canada ridge, but if it is too far west and close to Lee, could steer Lee eastward out to sea. Another weaker trough could enter from the west around this time and steer Lee to the northwest towards Maine or north/northeast toward Atlantic Canada depending on the relative locations of Lee and the trough. Additionally, the speed of Lee itself is somewhat uncertain. We will not have high confidence on this complicated setup until the middle of next week, with any impacts not until next Sunday or the following Monday. If Lee goes far enough west, it would provide heavy rain and strong winds to eastern New Hampshire and Maine, not too much unlike a strong, warm Nor’easter. If Lee goes farther east, there won’t be more than breezy conditions. In either case, Lee will be traversing cooler waters and therefore weakening as it approaches, though it will still be a powerful storm at our latitude given its size and original strength. However, it is unlikely to phase with any strong upper-level low to create a monster hybrid storm like Hurricane Sandy back in 2012.

 

 

Source: TropicalTidbits

 

Source: TropicalTidbits

 

With the frequent clouds and rain and high humidity through Wednesday, the diurnal temperature range will be small through then. Due to the trough axis and cut-off low to the west, areas northwest of the St. Lawrence Valley will be cooler, but still not particularly chilly for mid-September. After that, clearer skies and lower humidity will lead to a larger but still not particularly large diurnal temperature range. Also, with a lack of very hot or cold air masses for the foreseeable future, day-to-day temperature fluctuations will be smaller than usual. Most of the area will stay between 45F (7C) and 77F (25C) for the next 7-10 days! Despite the +PNA pattern causing a ridge in western North America and a trough in eastern North America, there isn’t any particularly cold air to pull down into our area with the troughs, and Hurricane Lee could inject heat from the tropics as well. There could be a drier and warmer period again the following week, but this is quite uncertain for now.

 

Source: TropicalTidbits

 

Source: Climate Prediction Center